Inorganic coating of luminescent porous silicon by ALD technique to enhance optical stability for biomedical applications

Porous silicon (pSi) is a sponge-like material produced by electrochemical etching of crystalline silicon wafer. The pSi particles gained a lot of interest in biomedicine because they are inert, biodegradable, biocompatible and have no immune response. They are proved to be photoluminescent (PL) due to quantum confinement effect and this is fundamental for bioimaging applications. One of the main problems of pSi particles for biomedicine applications is the fast quenching of the PL in biological solutions. Different approaches to solve this issue are present in literature: rapid thermal oxidation, as an example, leads to a PL stability of few days [Nature Commun., 2013, 4, 2326]. We previously demonstrated long-term stability of the optical properties by coating the pSi particles by organic layer such as chitosan and PEG [RSCAdv., 2017, 7, 6724]. In this work, we demonstrate the optical stability of the pSi particles in biological buffer (e.g. PBS) by depositing an inorganic TiO2 coating by means of Atomic Layer Deposition (ALD) in a rotary reactor [Surf. Coat. Technol. 2012, 213, 183]. The ALD method results in the deposition of a uniform coating on the particles and enables a fine tuning of its thickness [Chem. Soc. Rev. 2011, 40, 5242]. We investigated how the Ti precursor (TDMAT) and water vapour exposure time during ALD affect the optical properties of the pSi-TiO2 particles in biological solution. Based on our investigations, the water exposure time is a determining parameter for the PL properties. By optimizing the ALD parameters, the PL of pSi particles stabilized up to now more than three months without significant effect on the morphology. Quantum yield determination gave similar result for coated and uncoated particles. Optical stability makes pSi TiO2 coated particles a promising material for biomedical applications.